1. Field of the Invention
This invention relates to the alignment and switching of nematic liquid crystal devices.
2. Discussion of Prior Art
This invention relates to the alignment and switching of nematic liquid crystal devices.
Liquid crystal devices typically comprise of a thin layer of a liquid crystal material contained between cell walls. Optically transparent electrode structures on the walls allow an electric field to be applied across the layer causing a re-ordering of the liquid crystal molecules.
There are three known types of liquid crystal material nematic, cholesteric and smectic each having different molecular ordering. The present invention concerns devices using nematic materials.
In order to provide displays with a large number of addressable elements it is common to make the electrodes as a series of row electrodes on one wall and a series of column electrodes on the other cell wall. These form e.g. an x,y matrix of addressable elements or pixels and for twisted nematic types of device are commonly addressed using rms. addressing methods.
Twisted nematic and phase change devices are switched to an ON state by application of a suitable voltage and allowed to switch to an OFF state when the applied voltage falls below a lower voltage level, i.e. these devices are monostable. For a twisted nematic type of device (90xc2x0 or 270xc2x0 twist as in U.S. Pat. No. 4,596,446) the number of elements that can be rms. addressed is limited by the steepness of a device transmission verses voltage curve (as described by Alt and Pleschko in IEEE Trans ED vol ED 21, (1974) P.146-155).
One way of improving the number of pixels is to incorporate thin film transistors adjacent to each pixel; such displays are termed active switching element and include switching-elements such active matrix displays.
An advantage of nematic types of devices is the relatively low voltage requirements. They are also mechanically stable and have a wide temperature operating range. This allows construction of small and portable battery powered displays.
The main disadvantages of the above devices are as follows. The 90xc2x0 twisted nematic has a poor viewing characteristic which leads to loss of contrast when the device is viewed at high incident angles in certain azimuthal directions. Furthermore greyscale inversion occurs in these orientations. The low steepness of the 90xc2x0 twisted nematic can be improved by increasing the twist angle to 180xc2x0-270xc2x0. However this generally leads to no improvement in viewing characteristic. Both types of device also suffer from the fact that the large difference in the nematic tilt between the on and off states leads to a change in pixel capacitance which can cause crosstalk problems
According to this invention the above disadvantages are overcome using a surface treatment on one cell wall which treatment consists of an azimuthal alignment direction in a material or coated material which ordinarily induces a homeotropic orientation of the nematic director together with an alignment treatment on the other cell wall giving homogeneous alignment; the liquid crystal material has a negative dielectric anisotropy. This device may be called a VCT (voltage controlled twist device) configuration. It allows a steep optical response to be obtained with a greatly improved viewing characteristic. Furthermore only a small change in pixel capacitance occurs when the pixel is switched which leads to improved active matrix addressing and also less crosstalk in rms. addressing.
According to this invention a liquid crystal device comprises;
a nematic or chiral nematic liquid crystal of negative dielectric anisotropy
two containing walls, spaced apart and each so treated as to align the liquid crystal adjacent to them and carrying electrodes or other means to impose a field on the liquid crystal layer;
means for distinguishing between two different optical states;
Characterized by:
an aligning surface treatment at one cell wall providing a substantially planar or tilted alignment of the liquid crystal with a defined azimuthal first alignment direction;
an aligning surface treatment on the second cell wall capable of separately providing both a preferred, substantially homeotropic alignment of the adjacent liquid crystal, and a defined azimuthal second alignment direction to the adjacent liquid crystal dependant upon liquid crystal molecular arrangement;
the first and second azimuthal alignment directions being at a non zero angle to one another;
the arrangement being such that the liquid crystal material may adopt a twisted and a substantially non twisted molecular arrangement dependant upon applied electric fields.
The first and second azimuthal alignment directions may be substantially perpendicular or within 10xc2x0 of perpendicular. The polarisers optical axis may be orthogonal, or within about 10xc2x0 of being orthogonal. Additionally, the two polarisers may be rotated relative to the cell to obtain maximum contrast between the two states of the device.
The alignment at said second cell wall may comprise an alignment treatment ordinarily providing homeotropic alignment, in conjunction with a relief grating structure. The grating may be symmetric or asymmetric in profile, the groove depth and or pitch may be constant or may vary between pixels or within one pixel. Furthermore the grating groove direction may be constant or may vary between pixels or within one pixel. The grating surface may contain more than one modulation.
The alignment at said second cell wall may comprise an alignment treatment ordinarily providing homeotropic alignment, in conjunction with a rubbed polymer, or in conjunction with an evaporated inorganic layer such as MgF2.
The defined azimuthal alignment direction at said second cell wall may be provided by an anisotropic photoactive polymer layer.
The grating may be a profiled layer of a photopolymer formed by a photolithographic process e.g. M C Hutley, Diffraction Gratings (Academic Press, London 1982) p 95-125; and F Horn, Physics World, 33 (March 1993). Alternatively, the bigrating may be formed by embossing; M T Gale, J Kane and K Knop, J App. Photo Eng, 4, 2, 41 (1978), or ruling; E G Loewen and R S Wiley, Proc SPIE, 88 (1987), or by transfer from a carrier layer.
The planar surface may be a grating or a rubbed polymer or an anisotropically photopolymerised photopolymer or any other treatment which induces a substantially parallel (or a surface pretilt of typically 2 to 15xc2x0) orientation of the nematic director with respect to the surface with a preferred azimuthal direction.
The alignment on one or both walls may be formed by the technique of oblique evaporation of e.g. MgF2 etc.
The electrodes may be formed as a series of row and column electrodes arranged as an x,y matrix of rms. addressable elements or display pixels at electrode intersections. Typically the electrodes are 200 xcexcm wide spaced 20 xcexcm apart. The electrodes may be addressed by row and column driver circuits.
Alternatively, the electrodes may be arranged in other display formats e.g. r-xcex8 matrix or 7 or 8 bar displays.
The cell walls may be substantially rigid plates of glass. Alternatively, one or both walls may be of a flexible material, e.g. a plastics material such as POLYOLEFIN, PET.
The pixels may be addressed by an array of active switching elements, e.g. active matrix elements such as thin film transistors (TFT). In this case the TFT are formed on the surface of one cell wall and switched by e.g. x, y electrodes on the same cell wall; the second cell wall has formed thereon a single sheet or common plate electrode. One of these x,y electrodes is shaped into a pixel shape.
The means for distinguishing between two different optical states may be polarisers. Alternatively the means may be a small amount, e.g 1 to 5% of a dichroic dye included in the liquid crystal material together with one or two polarisers. Typical dyes are 2-4% of D102 (Merck), and about 4% of D6 (Merck).
Small amounts of a chiral dopant or cholesteric liquid crystal material, eg about 3% of C15 (Merck), may be added to the nematic liquid crystal material to impart a preferred twist direction.
The principle of operation is as follows. When a field is applied to the cell, the negative nematic material is forced to reduce its tilt angle until virtually the entire cell thickness is in a planar low tilt state with no twist. When the field is further increased, the planar configuration is forced closer to the homeotropic surface until the liquid crystal begins to interact with the alignment direction that is contained within the homeotropic surface. If this alignment direction is non parallel to the alignment direction of the opposite planar surface then this interaction will lead to twist. With suitable arrangements of polarisers, this twisting is accompanied by a change in optical transmission and hence the device acts as an optical switch.
The planar non twisted (off) state will appear dark if crossed polarisers are oriented parallel and perpendicular to the planar alignment direction. Furthermore this state appears dark when viewed from any direction. The twisted (on) state will appear bright. Therefore this display mode will possess a high contrast regardless of viewing direction: Also the xe2x80x98offxe2x80x99 to xe2x80x98onxe2x80x99 switching event occurs with minimal change in liquid crystal tilt angle therefore the capacitance does not undergo a large change.